The present invention relates to products useful in polishing optical surfaces. The surface polished can be glass or plastic.
It is well known that in order to produce a satisfactory optical surface, it is necessary that the surface be free of scratches and have as low an R.sub.a as possible. This R.sub.a measurement is the average distance between the highest and lowest points on the surface perpendicular to the plane of the glass sheet being polished. Thus, accepting that the surface will not be totally flat at the submicron scale, it is a measure of the variation between highest and lowest points. Clearly the lower the figure the better for optical clarity and freedom from distortion.
There is however another consideration and that is the speed at which the desired level of optical perfection is reached. Glass polishing is a chemical mechanical process that only proceeds in an aqueous environment. It is necessary for the polishing compound to react with the glass surface and the water, as well as the surface to be subject to abrasion. Some materials such as ceria are quite reactive but not very abrasive. Others such as alumina are quite abrasive but do not have much surface reactivity. This subject is well treated in an article by Lee Clark entitled "Chemical Processes in Glass Polishing" appearing in Journal of Non-Crystalline Solids 120 (1990), 152-171. In an industrial environment, there is a significant advantage in finishing the process in shorter rather than longer times, particularly when no quality sacrifice is required and or where quality can be improved.
In polishing processes there are two approaches. The first employs an abrasive formulation in the form of a slurry of the abrasives in a liquid medium. In the second the formulation is presented to the glass in the substrate to be polished in the form of a tool. This tool can be a bonded abrasive or a coated abrasive particularly one in which the coated abrasive surface is engineered to have properties that enable it to provide a very high uniformity of surface quality. It is with this latter approach that this invention is concerned.
In processes that use a slurry of abrasive particles in an aqueous medium, (usually based on deionized water), the slurry is placed in contact with the surface to be polished and a pad is caused to move across the surface in predetermined patterns so as to cause the abrasive in the slurry to polish the surface. In the second the abrasive particles are embedded in a resin matrix in the form of a tool and the tool is then used to polish the optical surface. The present invention relates to the first approach in which slurries are used.
U.S. application Ser. No. 09/025,730 U.S. Pat. No. 5,989,301 which was filed on Feb. 18, 1998 describes aqueous slurries comprising alpha alumina with a particle size of less than 0.5 micrometer and ceria in the form of a powder with sizes from 0.2 to 4 micrometers.
Tools comprising abrasive particles for optical polishing are somewhat limited by the requirement of excellent finish. While in theory bonded tools, that is to say tools in which the particles are retained in a bond material and the overall tool is given a predetermined shape such as a wheel or a stick, are not easily presented to the workpiece in a suitable form there have been developments in this field which now make this practicable. Coated tools are however recognized to be readily adapted to this application and coated tool products formulations comprising very fine solid particles retained in a binder that is a radiation-curable resin and deposited on a smooth substrate have proved very successful. Such coated tools have been described in for example U.S. Pat. No. 5,011,513 (Zador et al.) and U.S. Pat. No. 5,014,468 (Ravipati et al.). The Ravipati et al. patent represents a complete departure from traditional coated abrasives in which a substrate receives an abrasive grain either dispersed in a binder, (as in Zador et al.), or adhered by separately applied maker and size coats. In fact the workpiece is presented with an engineered surface comprising regular, similar, raised structures with void spaces between wherein the abrasive particles are very small by comparison with the size of the structures. This allowed the product to achieve excellent finishes quite quickly. Other later examples of engineered surfaces that can be used in the same fashion are described in, for example, U.S. Pat. No. 5,152,917 (Pieper et al.); U.S. Pat. No. 5,833,724; (Wei et al.) and U.S. Pat. No. 5,840,088 (Yang et al.).
Polishing tools are used in conjunction with a liquid medium sprayed on to the surface while the tool is polishing so as to provide the "chemical" portion of the chemical/mechanical polishing process, (sometimes abbreviated to "CMP").
Success in polishing glasses is of course to some extent dependent on the hardness of the glass. With very hard glasses polishing can take a very long time indeed and raises finish problems if the obvious expedient of using a harder abrasive is tried.
The formulations of the prior art are often very effective at achieving the desired result. However they also take quite a long time. A novel formulation has now been developed, where two oxides, "alumina and ceria", work together in synergy, such that their mutual interaction gives better results than the sum of any single component effects. This formulation permits a very high level of optical perfection to be achieved in a much shorter time than is attainable with such prior art formulations without the need for the elevated temperatures sometimes used to enhance reactivity. In addition they polish even hard glasses very effectively with little or no collateral damage to the surface. They can be used with "pad" or "pitch" type polishing apparatus or in polishing tools, particularly coated abrasives with engineered surfaces.